Enable VMs to be assigned to a standalone device without requiring a
cluster. Add device-scoped uniqueness constraints, update validation
logic, and enhance placement flexibility. Site is now auto-inherited
from the cluster or device.
* 7537 add serial number to virtual machines
* 7537 add migration
* 7537 add sn to search
* 7537 add to model documentation
* 8984 move serializer field
* 8984 add to detail view and search index
* 7537 serial_number -> serial
* 7537 fix migration
* Add missing serial field
* Give serial field higher precedence for search
---------
Co-authored-by: Jeremy Stretch <jstretch@netboxlabs.com>
